Posted: January 26, 2010

Where droplets gently touch a membrane

(Nanowerk News) Little or nothing happens in cells without the involvement of the minute membrane vesicles: for example, vesicles act as recycling centres for cell waste, as detoxification stations and as a vehicle for substance transport.
Scientists at the Max Planck Institute of Colloids and Interfaces have now described mathematically why some vesicles constrict to form a figure-eight shape. As part of this process, they have established that this constriction differs in its details to what was previously thought, and that its shape is dependent on the material characteristics of the components involved. This type of constricted vesicle may also play a role in vesicle formation in biological cells, as well as in the separation of cytoplasm into vesicles with different protein contents ("Intrinsic Contact Angle of Aqueous Phases at Membranes and Vesicles").
Membrane vesicles are usually spherical in form. However, they sometimes arise as double vesicles with the outline of a figure-eight - for example, when one vesicle separates off from another. "Viewed under an optical microscope, the two vesicles appear to be separated by a sharp incision," says Halim Kusumaatmaja, who played a major role in the study at the Max Planck Institute of Colloids and Interfaces in Potsdam-Golm. Researchers expect to find this in a double vesicle without a membrane. However, when a membrane surrounds the two vesicles, the constriction should curve to a greater or lesser extent. The fact that it does precisely this at the nanoscopic level is what the Potsdam researchers have succeeded in demonstrating with the help of theoretical and empirical tests.
The extent to which the constriction and the phase boundary curve depends on the components involved: the dissolved substances, the solvent and the characteristics of the membrane. This is where things become rather complicated, however, and without understanding how the double-bellied vesicles form, it is not possible to progress any further. The Potsdam researchers studied vesicles encapsulating a solution containing two polymers. Both polymers are very amenable to dissolving in water and try to stay out of each other’s way. The vesicles, in turn, float in another solution that is as strong as the solution found inside them. This is how things would remain if the researchers did not add an additional component to the exterior solution, which generates osmotic pressure: the exterior solution then contains more particles per litre than the solution inside the vesicles. As a result, water exits the vesicles through the membranes and enters the exterior solution.
While this process solves one problem, it creates a new one: the solution inside the vesicle becomes more concentrated due to the water loss - so strong that the two polymers, which are not very compatible, move closer to each other than they would like. As a result, the fluid in the vesicle divides into one part that contains more of one polymer and another with a higher proportion of the second polymer. Physicists refer to this phenomenon as phase separation. "It is possible that vesicles with different contents also form in cells in this way," says Halim Kusumaatmaja. In any case, the two fluids or phases in the vesicle are now separated from each other and from the fluid outside the vesicle.
The different chemical composition of the three solutions also gives rise to different physical behaviour - a different surface tension in this particular case. It determines the relative strength at the border between the two solutions and at the place where this interface touches the membrane. In exactly the same way that a drop of water assumes a more spherical form on a hydrophobic surface than it does on a hygroscopic one, on which it completely evaporates in extreme cases, the test of strength between the two fluids and the membrane, which is surrounded by a different fluid, causes the curvature of the vesicle borders.
The intrinsic contact angle, which arises from the characteristics of the components but cannot be measured, is a benchmark for the curvature. "We established a link between this contact angle and some measurable parameters," says Kusumaatmaja. Moreover, the scientists described mathematically how the form of the constriction arises from the forces involved, meaning the system’s quest for the minimum possible energy content.
"This system is of particular interest to us as it enables us to observe the interaction between wetting processes and membrane physics," says Reinhard Lipowsky who, as Director of the Potsdam-based Institute, heads the research. Wetting processes arise when a fluid comes into contact simultaneously with a gas or another fluid and a solid interface.
Circumstances change, of course, when the wetting of a flexible membrane is involved rather than that of a solid membrane. Even with two connected droplets with different contents floating in a third fluid without an enclosing membrane, the constricted membrane vesicles are not comparable: "In this kind of system, a sharp kink would actually arise between the droplets," says Halim Kusumaatmaja. "The gently curved constriction only arises with the membrane."
Based on the current findings, the scientists would now like to study how the shape of the vesicles depends on the intrinsic contact angle and the tension between the two solutions inside the vesicle. "We would like to fully understand the behaviour of such systems in order to learn even more about vesicle formation in biological cells", says Reinhard Lipowsky. This information could facilitate, for example, the handling of fluids on microchips in laboratories. Such minute analytical devices enable the implementation of sensitive and simple analyses in chemistry, biology and medicine. It may be possible to separate a fluid in this way into droplets with different contents, as the Potsdam-based researchers succeeded in doing in their basic investigations.
Source: Max Planck Institute of Colloids and Interfaces